The present invention relates to the communication of amplitude modulated (AM) signals over an optical communication path, and more particularly to the transmission of an amplitude modulated cable television spectrum over an optical fiber link.
Recently, there has been a growing interest in the development of analog optical communication systems. In comparison with digital systems, analog communication systems provide an efficient use of bandwidth. This is particularly useful in cable television (CATV) transmission system applications, where it is necessary to transmit a large number of video channels through an optical fiber. Compatibility with existing equipment is achieved by using the same amplitude modulated vestigial-sideband (AM-VSB) signal format for optical transmission that is in use for coaxial cable signal transmission.
The use of a television signal comprising AM-VSB video subcarriers is preferred in the distribution of cable television signals due to the compatibility of that format with NTSC television standards and the ability to provide an increased number of channels within a given bandwidth. An undesirable characteristic of AM-VSB transmission, however, is that it requires a much higher carrier-to-noise ratio (CNR) than other techniques, such as frequency modulation or digital transmission of video signals. Generally, a CNR of at least 40 dB is necessary to provide clear reception of AM-VSB television signals.
In order to transmit an information signal (e.g., a television signal) over an optical fiber, a light beam ("carrier") must be modulated with the information signal. Direct modulation of a laser providing the light source or external modulation of the laser beam are different approaches with respective advantages and disadvantages well known in the art. It is also known that with present technology, it is difficult to obtain semiconductor lasers that exhibit sufficient linearity and dynamic range for transmission of multiple channel amplitude modulated subcarrier video. Multi-channel CATV systems must provide on the order of 40 or more channels to remain competitive. A problem with multiple channel AM transmission is that the carrier-to-noise ratio and distortion product requirements of AM transmission are difficult to achieve in optical communication systems.
Frequency modulation (FM) has much lower CNR and distortion product requirements. Multiple channel video transmission has been demonstrated using individual subcarriers for each of a plurality of FM modulated video channels. This technique of FM subcarrier modulation for video distribution is costly, however, since each channel requires its own modulator at the transmitter and demodulator at the receiver. Further, in order to achieve compatibility with existing cable systems, the recovered video in such an FM transmission scheme must be frequency translated to the proper television channel allocation. This requires a complex system that uses a large number of components.
It would be advantageous to provide a method and apparatus that takes advantage of the lower channel CNR and distortion product requirements of FM transmission, while maintaining the compatibility advantages of AM signal distribution. It would be further advantageous to provide a receiver for receiving signals communicated using such a technique. The present invention provides such a method and apparatus.